This invention relates to an analog-type electronic wristwatch structure equipped with a flat battery, such as a so-called coin-type lithium battery, having a large energy capacity per unit volume and a large diameter in comparison with the thickness thereof (i.e., a large planar area).
Analog-type crystal wristwatches have achieved popularity with comparative rapidity in recent years. These wristwatches are composed of such components as a crystal oscillator circuit serving as a time base oscillator, a frequency divider circuit which divides a high-frequency time base signal, produced by the oscillator circuit, down to a low-frequency unit time signal, a driver circuit which produces drive signals upon receiving the unit time signal produced by the frequency divider circuit, an electro-mechanical transducer, such as a stepping motor, driven in response to the drive signal from the driver circuit, a mechanical transmission mechanism such as a wheel train driven by the electro-mechanical transducer, hands which are advanced in accordance with the operation of the mechanical transmission mechanism, time correction means for correcting the time displayed by the hands, and a power supply battery for supplying the oscillator circuit, frequency divider circuit, driver circuit and electro-mechanical transducer with electrical energy.
The conventional analog-type crystal wristwatches employ a so-called button-type silver battery (silver oxide or silver peroxide) as the power supply battery. The button-type silver battery has a smaller energy capacity per unit volume in comparison with the coil-type lithium battery mentioned above, and therefore has a short lifetime of, in general, two to three years. This means that the battery must be replaced in a comparatively short period of time. In addition, since the button-type silver battery itself has a construction that is prone to leaking an electrolyte, major failures may occur in which such portions as the timepiece circuitry and wheel train malfunction owing to leakage of the electrolyte from the battery. These are some of the defects that can be cited in connection with the button-type silver battery.
More specifically, while it is true that analog-type crystal wristwatches have rapidly gained popularity, it is also a fact that such popularity is limited to specific regions such as the commercially and industrially advanced countries typified by North America, Europe and Japan, which have a comparatively high population density as well. In certain other regions, however, where servicing (market servicing) networks are incomplete and where replacement batteries cannot be readily obtained, the spread of wristwatches that rely upon batteries has never been as great as hoped owing to the two-to-three year battery replacement cycle.
Furthermore, the button-type silver batteries generally make use of strongly alkaline electrolytes such as KOH or NaOH. These strongly alkaline electrolytes are such that they creep upwardly along the inner wall of the battery container and are-quite hazardous in that the sealed mouth of the battery may be deformed by a build-up in the pressure of the gases evolved as chemical changes within the battery progress. These factors greatly raise the possibility of leakage from a battery installed within the wristwatch and therefore contribute to a high incidence of malfunction caused by such leakage. Button-type silver batteries are particularly likely to leak when they are left within the wristwatch for a long period of time following the end of their life. Accordingly, in regions where it is difficult to replace the battery soon after the end of its life, the question of popularizing wristwatches that rely upon the button-type silver battery is related directly to the higher incidence of malfunction caused by leakage. It is obvious that this is a major problem in connection with servicing as well.
Thus, great import is to be found in diminishing the frequency of the battery replacement operation by extending the life of such batteries used in wristwatches, and in diminishing the frequency of leakage-induced malfunctions by checking battery leakage. The reason is not only that such achievements are vital for popularizing crystal wristwatches everywhere by dispelling the misgivings associated with the use of batteries, but that they enhance timepiece reliability, durability and servicing even in areas where crystal wristwatches are already widespread.
Meanwhile, so-called coin-type lithium batteries have recently won attention as power supply batteries for miniature electronic devices such as digital display-type electronic wristwatches and electronic desk-top calculators. These coin-type lithium batteries have a much greater energy capacity per unit volume than the button-type silver batteries and, even if used as the power supply in an analog-type crystal wristwatch, would assure a battery life of at least five years and, in general, of from 7 to 10 years. Moreover, they employ organic electrolytes, including propylene carbonate or .gamma.-bulyrolactone, which leak from the battery only with great difficulty. Accordingly, a lithium battery left in a timepiece, even for a long period of time, will leak only rarely, and any leakage that does occur will be very slight. Therefore, if it were possible to use a lithium battery as the power supply in an analog-type crystal wristwatch, malfunctions caused by battery leakage would obviously be prevented from occurring.
Lithium batteries thus possess such advantages as a long lifetime owing to a great energy capacity, and a very low probability of leaking. Nevertheless, analog-type electronic timepieces that employ the lithium battery as a power supply have not as yet been realized. To be more specific, since a lithium battery tends to undergo an increase in internal resistance as the battery is increased in thickness, a lithium battery for a wristwatch should have a coin-like configuration, that is, a comparatively small thickness and a diameter which is sufficiently large in comparison with the thickness. Thus, if we were to imagine a coin-type lithium battery for use in a wristwatch, the battery would have a diameter of from about 16 to 25 millimeters, a thickness of 0.6 to 2.5 millimeters, and a weight of 0.8 to 4.0 grams. However, if we now take into consideration the fact that the diameter of the main plate in a men's wristwatch has a diameter on the order of from 23 to 28 millimeters, then we are confronted with the problems of how to dispose and support the coin-type lithium battery within an analog-type electronic wirstwatch. Thus it may be understood that employing such a battery in an analog wristwatch is extremely difficult in actuality as long as the aforementioned problems remain unsolved. More specifically, electronic wristwatches having an analog display are different from those having a digital display in that they include the wheel train driven by the electro-mechanical transducer. The wheel train is disposed close to the approximate center of the main plate when viewed in plan, and between the main plate and the wheel train bridge when viewed in cross-section. If we take note of the relationship between the respective diameters of the main plate and the coin-type lithium battery, it will be obvious that the coin-type lithium battery will necessarily overlap other components in view of the space occupied by the wheel train and wheel train bridge in an analog-type electronic wristwatch. This leads to two problems that must be solved simultaneously, namely (1) how to arrange, in their entirety, the elements that constitute an analog-type electronic wristwatch so as to minimize the increase in wristwatch thickness and the complexity of the wristwatch structure, and (2) how to support the coin-type lithium battery so as to prevent the battery from transmitting an impact force directly to the movable wheel train when such a force, arising from external causes, acts upon the wristwatch.
To develop this point further, a wristwatch is a commodity which increases in value as the thickness thereof decreases. When employing a coin-type lithium battery as described above, the ratio of the diameter d of the battery to the diameter D of the main plate is given by the expression 0.65.ltoreq.d/D.ltoreq.0.95, so it is inevitable that the battery will overlap a majority of the other elements when it is installed. This necessitates an arrangement specifically conceived so as to avoid any increase in wristwatch thickness. And again, unlike digital-type electronic wristwatches, analog-type electronic wristwatches have movable parts, such as the wheel train and a portion of the electro-mechanical transducer, that are supported by subminiature structures so as to be capable of rotation and other forms of mechanical operation. If a heavy coin-type lithium battery is so disposed as to overlap the movable parts, then it is absolutely essential to avoid an arrangement in which the movable parts are in danger of being damaged by a directly received impact load ascribed to the existence of the battery. It is of course possible to adopt a special structure for the back cover which constitutes a portion of the wristwatch case, and to retain a flat-type battery within this back cover itself, or to dispose a second back cover between the timepiece movement and the first back cover, with the flat-type battery being accommodated and retained between the first and second back covers. Such arrangements as these have already been proposed. Even if such arrangements do prevent the battery from subjecting the movable parts to an impact load such as may result from an impact experienced when the wristwatch is dropped, they nevertheless complicate the overall structure and raise the cost, and greatly increase wristwatch thickness. The state of the art is such that these arrangements have not been adopted in actual products.